The Epidemiology Branch (DESPR) has conducted a case-control study in Illinois and California to determine whether the use of periconceptional vitamin supplements can reduce the risk of neural tube defects. Women having either a fetus or an infant with a neural tube defect have been ascertained through perinatal networks, vital records, and other sources and were matched to two controls on maternal race and geographic locale. One control is a mother with a normal pregnancy, and the other the mother of an infant with a fetus with a major health problem other than a neural tube defect. Cases and controls were interviewed within 3 months of the end of pregnancy to determine whether those having a conceptus with a neural tube defect are less likely to have used vitamins in the periconceptional period. A second major objective was to determine whether or not fertility drug use is associated with neural tube defects. Data on infertility and the use of ovulation-inducing drugs were collected during the same interview. Additional analysis has examined the effects of extreme obesity.